There are a lot of possibilities....
Mounting a scope should be done with a torque screwdriver. I should know. Before I figured this part out, I mounted a lot of them wrong. I cringe at some of my early work.
Base screws should be tightened to spec, with a drop of blue Locktite. In many cases, if you tighten the screws to spec without Locktite, they will quickly work their way loose.
Make sure the front base screw is not bottoming out against the barrel before the base is secured. If it is bottoming out, you'll find bluing rubbed off at the edges of the base. Grind a bit off the tip of the screw.
If you are using aluminum rings, there is very often a simple problem. About the time the ring mounting screws are feeling snug, the head of the mounting bolt on the other side of the base pulls part way through the aluminum. (Look for recessed heads.) Once that happens, you'll never get good groups until you fix it.
The fix is to remove the mounting bolt entirely, coat the splined head with JB Weld, and hold it in place with gentle tightening. After the epoxy sets, do not exceed the recommended torque. Use blue Locktite on the threads to make sure it doesn't move.
The action screws might be loose.
The scope might be dead.
A malevolent genie may have simply decided to urinate on your day.